In recent years there has been a renewed interest in alternatives to petroleum-based fuels. The alternative fuels must be technically acceptable, economically competitive, environmentally acceptable and easily available. The need for these fuels arises mainly from the standpoint of preserving global environment and concern about long-term supplies of conventional hydrocarbon based fuels. Among the different possible sources, diesel fuels derived from trigycerides (vegetable oil/animal fat) present a promising alternative. Although triglycerides can fuel diesel engines their viscosities and poor cold flow properties have led to investigation of various derivatives. Fatty acid methyl esters derived from trigycerides and methanol known as bio-diesel, have received the most attention. Vegetable oils are widely available from a variety of sources. Unlike hydrocarbon-based fuels, the sulfur content of vegetable oils is close to zero and hence the environmental damage caused by sulphuric acid is reduced.
The main advantages of using bio-diesel are its renewability, better quality exhaust gas emission, its biodegradability and given that all the organic carbon present is photosynthetic in origin, it does not contribute to a rise in the level of CO2 in the atmosphere and consequently to the greenhouse effect. Several processes for transesterification of triglycerides have been developed: (1) Base-catalyzed transesterification of glycerides with alcohol (catalysts—alkaline metal alkoxides and hydroxides as well as sodium and potassium carbonates), (2) Direct acid-catalyzed esterification with alcohol (catalysts—Brönsted acids, preferably sulfonic acid and sulfuric acid), and (3) Conversion of oil to fatty acids and then to alkyl esters with acid catalysis. However, the former route (i.e., base catalyzed reaction) is the most economical and in fact, is in practice in several countries for bio-diesel production (J. Braz. Chem. Soc. Vol. 9, No. 1, Year 1998, pages 199-210; J. Am. Oil. Chem. Soc. Vol. 77, No. 12, Year 2000, pages 1263-1266; Fuel Vol. 77, No. 12, year 1998, pages 1389-1391; Bioresource Tech. Vol. 92, Year 2004, pages 55-64; Bioresource Tech. Vol. 92, Year 2004, pages 297-305; Renewable Sustainable Engery Rev. Vol. 9, Year 2005, pages 363-378). Alkaline metal alkoxides (as CH3ONa for the methanolysis) are the most active catalysts, since they give very high yields (>98%) of fatty acid alkly esters in short reaction times (30 min) even if they are applied at low molar concentrations (0.5 mol %) (J. Food Composition and Analysis Year 2000, Vol. 13, pages 337-343). However, they require high quality oil and the absence of water, which makes them inappropriate for typical industrial processes (J. Braz. Chem. Soc. Vol. 9, No. 1, Year 1998, pages 199-210). Alkaline metal hydroxides (NaOH and KOH) are cheaper than metal alkoxides but require increasing catalyst concentration (1-2 mol %). NaOH is more superior to KOH as the latter and other alkali hydroxides yield more soponified products than the bio-fuel.
Recently, enzymatic transesterification using lipase has become more attractive for bio-fuel production, since the glycerol produced as a by-product can easily be recovered and the purification of fatty acid esters is relatively simple to accomplish. However, the main hurdle to commercialize this system is the cost of lipase production (J. Mol. Catal. B: Enzymatic Vol. 17, Year 2002, pages 133-142).
Use of immobilized lipases in the synthesis of fatty acid methyl esters from sunflower and soybean oils were reported by Soumanou and Bornscheuer and Watanabe et al (Enzy. Microbiol. Tech. Vol. 33, Year 2003, page 97; J. Mol. Catal. B: Enzymatic Vol. 17, Year 202, pages 151-155). They found that the immobilized enzyme is active at least for 120 h during five batch runs without significant loss of activity. Among the various lipases investigated the enzyme from Pseudomonas fluorescens (Amano AK) exhibited the highest conversion of oil. Khare and Nakajima (Food Chem. Vol. 68, Year 2000, pages 153-157) also reported the use of immobilized lipase enzyme.
Cost is the major factor slowing the commercialization of biofuels. Replacement of homogeneous catalyst by a solid catalyst eliminates the processing costs associated with the homogeneous catalysts. Leclercq et al. (J. Am. Oil. Chem. Soc. Vol 78, Year 2001, page 1161) studied the transesterification of rapeseed oil in the presence of Cs-exchanged NaX and commercial hydrotalcite (KW2200) catalysts. At a high methanol to oil ratio of 275 and 22 h reaction time at methanol reflux, the Cs-exchanged NaX gave a conversion of 70% whereas 34% conversion was obtained over hydrotalcite. ETS-4 and ETS-10 catalysts gave conversions of 85.7% and 52.7%, respectively at 220° C. and 1.5 h reaction time (U.S. Pat. No. 5,508,457). Suppes et al (J. Am. Oil. Chem. Soc. Vol. 78, Year 2001, page 139) achieved a conversion of 78% at 240° C. and >95% at 160° C. using calcium carbonate rock as catalyst. Of late, Suppes et al reported the use of Na, K and Cs exchanged zeolite X, ETS-10, NaX occluded with NaOx and sodium azide in the transesterification of soybean oil with methanol (Appl. Catal. A: Gen. Vol. 257, Year 2004, page 213). Furuta et al (Catal. Commun. Vol. 5, Year 2004, pages 721-723) describe biodiesel production from soybean oil and methanol at 200-300° C. using solid superacid catalysts of sulfated tin and zirconium oxides with oil conversions over 90%. Use of tin complexes immobilized in ionic liquids for vegetable oil alcoholysis was reported by Abreu et al (J. Mol. Catal. A: Chem. Vol. 227, Year 2005, pages 263-267; J. Mol. Catal. A: Chem. Vol. 209, Year 2004, pages 29-33). Kim et al reported the use of heterogeneous base catalysts (Na/NaOH/Al2O3) for the methanolysis of vegetable oils
U.S. Pat. No. 5,713,965 describes the production of biodiesel, lubricants and fuel and lubricant additives by traneseterification of triglycerides with short chain alcohols in the presence of an organic solvent such as an alkane, arene, chlorinated solvent, or petroleum ether using Mucor miehei or Candida Antarctica-derived lipase catalyst. Patents Nos. WO 00/05327 A1, WO 02/28811 A1, WO 2004/048311 A1, WO 2005/021697 A1 and WO 2005/016560 A1 and U.S. Pat. Nos. 5,578,090; 6,855,838; 6,822,105; 6,768,015; 6,712,867; 6,642,399; 6,399,800; 6,398,707; 6,015,440, also teach us the production fatty acid alkyl esters using either lipase catalysts or metal ion catalysts. Patent No. WO 2004/085583 A1 describes transesterification of fats with methanol and ethanol in the presence of a solid acid catalyst having ultrastrong-acid properties in a short time at around ordinary pressure.
Production of diesel from pure soybean oil or coconut oil is not economical, so it is desirable to use cheaper alternative feedstocks such as animal fat or used cooked oil or oil from seeds of wild plants like jojoba and jatropha. Animal fat and used oil contain high amounts of free fatty acids (FFA) content. The FFA saponifies with the alkali-based transesterification catalyst leading to low yield, difficulties in separation of the products, and increase in production cost. In those cases a two step process wherein in the first step an acid catalyst esterifies the free fatty acids to methyl esters and in the second step a base catalyst transesterifies the triglycerides is generally employed in diesel preparation. An efficient solid catalyst, which can do this in a single-step is highly desirable.
The present invention deals with a process, which eliminates most of the above said drawbacks. It deals with production of hydrocarbon fuels (diesel oil) which comprises reaction of vegetable oils or fats with C1-C5 alcohols at moderate conditions using a novel, solid, reusable double metal cyanide catalyst. The feedstock oil is a triglyceride or a mixture of fatty acids and glycerides. One of the metals of the double metal cyanide catalyst is Zn2+ while the other is a transition metal ion preferably Fe. Co-existence of Zn and Fe in the active site linking through cyano bridges makes it efficient to transform feedstocks containing fatty acids in a single step to fatty acid alkyl esters. The catalyst could be separated easily by centrifugation or by simple filtration and reused. Most importantly, the catalyst is highly efficient and only a small amount (˜1 wt % of oil) is needed to carryout the reaction. The process is atom-efficient and the reaction conditions like temperature and pressure are only moderate. Unlike the conventional base catalysts the catalyst of the present invention is more efficient even in the presence of water impurity in oil. Hence, there are no limitations on the quality of oil that should be used with the catalysts of the present invention.